(1) Field of the Invention
This invention relates to a method of Optical Proximity Correction for photolithographic patterns and more specifically to the use of mathematical modeling based on curve fitting to achieve Optical Proximity Correction.
(2) Description of the Related Art
Photolithography is critical to the fabrication of integrated circuit wafers and utilizes masks to transfer images, such as line/space patterns, to the wafer. As circuit densities increase critical dimensions decrease and line/space patterns become more and more dense. FIGS. 1A and 1B show an example of line distortion that can occur for small critical dimensions, such as in the sub-micron range. FIG. 1A shows an intended pattern element 10 which is to be transferred to a layer of resist material on an integrated circuit wafer. FIG. 1B shows the optical effects of the small dimensions in the patterns 11 actually transferred to the layer of resist. As shown in FIG. 1B the corners of the pattern are rounded resulting in rounded ends 12 of the pattern element and a shortened line. The dashed lines 14 indicate the intended limits of the pattern element.
FIGS. 2A and 2B show an example of corner rounding that can occur in pattern segments of larger rectangular regions. FIG. 2A shows an intended pattern of a larger rectangular pattern segment 16 which is to be transferred to a layer of resist material on an integrated circuit wafer. FIG. 2B shows the optical proximity effects on the corners of the larger rectangular pattern segment 17 actually transferred to the layer of resist. As shown in FIG. 2B the corners of the pattern segment are distorted resulting in rounded corners 19. The dashed lines 18 indicate the intended limits of the pattern segment.
This optical distortion shown in FIGS. 1A-2B is the result of Optical Proximity Effect. Optical Proximity Effect is a form of optical distortion associated with the formation of photolithographic images. Diffraction effects occurring on both sides of a sharp pattern edge become important as the critical dimensions of pattern features decreases. Optical Proximity Correction is employed to compensate for the Optical Proximity Effect.
U.S. Pat. No. 5,553,273 to Liebman describes a method which first identifies a plurality of design shapes in computer aided design data for a lithography mask. The design shapes are sorted according to geometric type. The design shapes are them sorted and a bias is applied to the sorted design shapes.
U.S. Pat. No. 5,182,718 to Harafuji et al. describes a method of correcting design patterns in cells having hierarchial structure and corresponding to exposure patterns.
U.S. Pat. No. 5,432,714 to Chung et al. describes a system and method for preparing shape data for proximity correction.
U.S. Pat. No. 5,827,623 to Ishida et al. describes a method of forming an improved halftone type phase shift mask having an Optical Proximity Correction function.
U.S. Pat. No. 5,682,323 to Pasch et al. describes a system and method of performing optical proximity correction on an integrated circuit by performing optical proximity correction on a library of cells.
U.S. Pat. No. 5,723,233 to Garza et al. describes a photolithography Optical Proximity Correction method for mask layouts.
A paper by O. W. Otto et al. "Automated optical proximity correction--a rules-based approach," Optical/Laser Microlithography VII, Proc. SPIE (2197) 1994, pages 278-293 describes a rules based approach for optical proximity correction.
A paper by J. Stirniman and M. Rieger "Optimizing proximity correction for wafer fabrication process," 14th Annual BACUS Symposium on Photomask Technology and Management, Proc. SPIE (2322) 1994, pages 239-246 describes a methodology for characterizing proximity effects from measurements taken on a processed wafer.
A paper by S. Shioiri and H. Tanabe "Fast Optical Proximity Correction: Analytical Method," Optical/Laser Microlithography VIII, Proc. SPIE (2440) 1995, pages 261-269 describes a method for calculating proximity corrected features analytically.